There is a loose hierarchy of the cosmos that repeats.  Stars form clusters, and then galaxies.  Galaxies form clusters, and then these form clusters of their own, called superclusters.  Gravity dominates the structure of such collections, yet all we feel and see from Earth is a relatively homogeneous distribution of stars.  How do we see this hierarchy?  If we zoom in, looking at a patch of sky so tiny that we can’t see any stars, what do we see?  This patch of sky is about the size of a grain of sand held at arm’s length, and if we point the...